1. Field of the Invention
The present invention relates to a processing aid for polymers, and to polymers having improved processibility, particularly improved extrusion characteristics.
2. Background
The melt extrusion of high molecular weight polymers, for example, hydrocarbon polymers, into shaped structures such as tubing, pipe, wire coating or film is accomplished by well-known procedures wherein a rotating screw pushes a heated, molten and viscous polymer melt through the extruder barrel into a die in which the polymer is shaped to the desired form and is then subsequently cooled and resolidified, by various means, into the general shape of the die.
In order to achieve low production costs it is desirable to extrude at high rates. Although the extrusion rate is readily increased by increasing the rate of revolution of the extruder screw, there is a technical limit to these increases because of the viscoelastic properties of the polymer. At rates above this limit the polymer may be mechanically heated to temperatures at which thermal decomposition can occur, or extrudates with a rough surface are obtained. The latter phenomenon can generate an undesirable pattern on the surface of the extrudate. One way of avoiding this occurrence is to extrude at a higher temperature, but this adds to the processing costs and makes cooling of the extrudate more difficult. More seriously, many polymer resins are already extruded at temperatures near their decomposition temperatures, and further increases are not feasible.
Additional examples of poor extrusion behavior include formation of deposits of the extruding polymer resin, degraded polymer resin or resin additives around the die exit (orifice); surging in polymer flow and erratic pressure fluctuations that occur due to uneven pumping of the polymer melt, which may result in erratic fluctuations of output rate; and torquing out of the extruder, that is, overloading the pressure limit of the extruder, resulting in automatic shutdown of the extruder.
It is desirable, therefore, to find highly efficient means of increasing the extrusion rate, without raising the melt temperature, while producing products with smooth surfaces. Changes in extruder and die configuration can improve melt flow but are not always practical or economically feasible. Another approach involves the addition of conventional wax-type process aids which reduce bulk viscosity and in some cases improve processing properties. However, the efficiency is marginal and the high levels of additive required often adversely affect other properties. In Blatz, U.S. Pat. No. 3,125,547, it is disclosed that the use of 0.01-2.0 wt. % of a fluorocarbon polymer that is in a fluid state at process temperature, such as a fluoroelastomer, will reduce die pressure and significantly increase the extrusion rate at which melt fracture occurs for high and low density polyethylenes and other polyolefins. It is further taught in U.S. Pat. No. 3,125,547 that fluororesins which are solids at process temperature afford little or no improvements in extrusion characteristics of hydrocarbon polymers. By the term solid, it is meant that the fluororesin, if crystalline in nature, is not melted, or, if amorphous in nature, is not above the glass transition temperature.
Kamiya and Inui, in Japanese Patent Application Publication Kokoku 45-30574 (1970, examined) cite the use of crystalline fluorocarbon polymers at temperatures below their melting points to eliminate die build-up but say nothing of other extrusion improvements. Nishida, Tate and Kitani, in Japanese Patent Application Publication Kokai 62-64847, disclose injection molding compositions comprising an ethylene/alpha olefin copolymer having an MFR of 0.2-200 g/10 min., a density of 0.850-0.945 g/cm.sup.3, and a Q value of 2.5-12, and 0.001-1% by weight of a fluorinated hydrocarbon polymer having an F/C ratio of at least 1:2.
Chu, in U.S. Pat. No. 4,740,341, discloses blends having improved extrudability and comprising a linear polymer of ethylene having incorporated therein 0.01-0.5 wt. %, based on the composition, of a fluorocarbon polymer having an F/C ratio of at least 1:2 and which is fluid at 120-300.degree. C., and 0.01-0.5 wt. %, based on the composition, of a polysiloxane.
Larsen, in U.S. Pat. No. 3,334,157, discloses polyethylene which has been modified to improve its optical properties by incorporating therein 0.015 to greater than 1.7% by wt., based on the mixture, of finely divided polytetrafluoroethylene.
It is an object of this invention to provide resin compositions with substantially improved extrusion characteristics. It is another object to provide polymers which can be extruded at high rates to give extrudates of high surface quality. It is yet another object to provide polymers that can be extruded at low die pressures and at low melt temperatures. Another object is to provide a processing aid by means of which all the above can be achieved. A further object is to provide masterbatches of the processing aid. Other objects will become apparent hereinafter.